1. Field of the Invention
The present invention relates to a heat-transfer material utilized for example as a condenser tube or an evaporator tube of a heat exchanger for use in an air conditioner, or as a heat pipe, and to a method of producing the same.
2. Related Art
Several effective ways to increase the efficiency of heat-transfer in a heat-transfer tube are generally known: (1) increasing the heat-transfer area; (2) causing a turbulent flow; (3) causing capillarity; and (4) causing nucleate boiling. As a heat-transfer tube of which efficiency of heat-transfer is improved by the above-mentioned ways (1) and (2), a copper tube having spiral grooves formed in an inner periphery thereof is conventionally employed. However, when rolling the spiral grooves in the inner periphery of the tube by a rolling apparatus, the number and helix angles of the grooves are restricted due to the restrictions on the techniques of rolling operation and of making the rolling tools. As a result, the efficiency of heat-transfer for the grooved tube can be increased to a level of only 1.2 to 1.5 times that of a tube with no grooves, thereby being not sufficient. In addition, a great force is required to roll the grooves in the manufacture of the grooved tube since great friction is exerted between the rolling tool and the inner surface of the tube. Accordingly, a large rolling apparatus is required, and besides the service life of the tool is short, thereby increasing the manufacturing cost.
Further, as a heat-transfer material improved by the above-mentioned way (4), which way is considered to be most effective, a material of a metal having a porous metal layer formed on a surface thereof by a sintering method or a brazing method is known. However, although the porous layer can be easily formed by means of sintering or brazing for a plate-like like heat-transfer material, it has been difficult to form such a porous layer on the inner surface of a tubular member such as a heat-transfer copper tube by the method. Furthermore, electroplating can be employed to form the porous layer on a surface of a metal after the step of effecting pattern masking on the metal surface by screen process printing. The method, however, can not be employed to form the porous layer on the inner periphery of the tube either, and besides requires complicated steps such as printing, thereby increasing the manufacturing cost substantially.